I have same exact problem as this guy .
https://forums.tomshardware.com/threads/insane-ping-spikes-only-in-rainbow-six-siege.3530132/: (https://forums.tomshardware.com/threads/insane-ping-spikes-only-in-rainbow-six-siege.3530132)
here is what he said:
The game runs amazingly at 130fps+ (mine is 60fps but can manage to 80 or more) with no stutters or lag. What happens is when I am in game my internet connection goes down significantly. It becomes unplayable with 200+ ping and can no longer hear people in Discord. It only happens in this game.. Tried Valorant,CS:GO,League of Legends and it works flawlessly, no connectivity problems. (For Ubisoft Support Team i can send them DxDiag file and MSI info file)
Thank you.